Carbon nanotubes (NTs) and graphene have tremendous potential for future nanoscale applications due to their remarkable physical properties, such as high carrier mobility and mechanical strength. To realize the potential of these two closely related materials, both comprising sp2-bonded honey-comb structured carbon sheets, significant improvements to the crystallographic control over their construction, orientation, and placement at the nanoscale are required. Towards this goal, a number of techniques have been utilized to precisely control the orientation and placement of NTs, such as aligned growth utilizing atomic step-edge templates, single-crystal templates, flow-alignment, electric-field alignment, and combinations of the above techniques yielding novel structures such as serpentines. Yet the growth of nanotubes with specific chirality has still been a major technological obstacle. Another remaining obstacle has been the growth of nanotubes on electrically conducting substrates.
Crystallographic nanoscale control over the construction of graphene and few-layer graphene (FLG) structures has seen progress through nano-lithographic methods, crystallographic catalytic etching, etch masks made from nanowires and metallic nanojunctions, localized etching with scanning probes, ultrasonication, and plasma etching of NTs. Additional recent efforts in precision nanofabrication have been directed towards the goal of directly growing NTs from carbon sources, such as graphene oxide and reduced graphene oxide, without the need of a feedstock gas. Although NTs have been shown to have a chirality dependent adhesion to graphene, most previous investigations focused on the interactions between NTs and graphene in its bulk graphite form; with some of this work showing that NTs grown through laser ablation can be oriented along specific bulk graphite crystal axes.
This document discloses the growth of NTs on electrically conducting few-layer graphene (FLG) films using catalytic chemical vapor deposition (CVD). We find that NTs grow along specific crystallographic orientations of the FLG films. Moreover, this crystallographic orientation becomes significantly more pronounced on thinner films that are less than approximately 6 atomic layers thick. The orientations of the NTs are 30° offset from crystallographic etch directions occurring in graphene, indicating that the NTs lie along the armchair directions of the FLG lattice. A striking feature of the NTs on FLG is that they make occasional abrupt 60° or 120° changes in direction along the other armchair orientations. These abrupt changes in crystallographic direction also occur when NTs encounter one another, which is indicative of a tip-growth mechanism along the surface of the FLG.